This invention relates to hub clutches for the front wheels of four-wheel drive vehicles and more particularly to an improved actuating mechanism for such clutches.
In four-wheel drive vehicles, it is sometimes desirable to have all four wheels fixedly connected to their drive axles, as for example, when the vehicle is traveling over difficult terrain. However, it is also advantageous to be able to disengage the front wheels or at least permit them to overrun the axle when the vehicle is traveling on a paved or smooth road surface.
Hub clutches for automotive vehicles are well known and are designed for use on light and medium weight vehicles equipped with front driving axles. The hub clutch enables the front wheels to be disengaged from the front drive line when front wheel drive is not desired. Hub clutches allow the front wheels to rotate without "back drag" from the front axle and propeller shaft when four-wheel drive is not needed. This eliminates unnecessary wear and results in greater fuel economy.
The prior art discloses a number of wheel locking hubs which permit the front wheels of a four-wheel drive vehicle to operate in both locked and unlocked modes. Hub clutches of this type are selectively operable between the automatic and normal modes by manual movement of a selector which moves a shiftable clutch member axially into or out of positive locking engagement with an axially fixed clutch member.
A cam is required to translate the rotational movement of the selector into axial movement of the shiftable clutch member. The axial movement enables the shiftable clutch member to engage the fixed clutch member, when the selector is rotated in one direction thereby locking the associated wheel to the front axle. When the selector is rotated in the opposite direction the shiftable clutch member slides axially out of engagement with the fixed clutch member, thereby freeing the wheel from the axle shaft. One of the more common methods of achieving this camming action is by providing an actuator or a nut which is screwed on a helically threaded portion of the selector. Another well known method of accomplishing the camming action is by fixing a cam member to the selector and providing a cam follower plate for engaging and moving the shiftable clutch member in opposite directions.
In either instance, because of the possibility of mis-alignment between the clutch members when being engaged or due to presence of excessive torque wind-up when engaged, a spring is often provided for engaging the clutch members. For example, if the clutch members are moved to their "lock" position and are circumferentially off-set with relation to each other, the spring becomes compressed and continues to bias the clutch members toward their engaged position. Upon relative movement between the wheel and axle, the spring will snap the clutch members into their engaged position. On the other hand, if the clutch members are in their engaged position and the selector is moved to the "free" position with the clutch members being restrained due to torque wind-up, the spring is placed in tension. As soon as the vehicle is moved slightly, the spring will withdraw the clutch members thereby disengaging the clutch members.
It is advantageous under the above circumstances to have a single resilient means such as a spring capable of acting both in tension and compression. These double acting springs are often used to move the shiftable clutch member and the follower portion of the camming mechanism. However, these clutches require excessive axial space during the clutch shifting operation because the spring must act in an axial direction under both tension and compression. Furthermore, because the springs are located outwardly of the selector, they are more readily susceptible to damage since they are directly associated with the shifting clutch member of the locking hub. Thus, while these mechanisms operate in a satisfactory manner, they are complex, more difficult to assemble and more expensive due to the number of components required to carry out the desired operation.